Excessive B cell activity and loss of regulatory cells occurs in patients with active SLE; these abnormalities are less marked when the disease is quiescent. Nevertheless, excessive B cell proliferation occurs even in inactive patients. Loss of a subpopulation of regulatory T cells in active SLE is caused by antibodies to these, spontaneously produced by the patients. Such antibodies prevent normal T cell control of abnormal B cell activity allowing perpetuation of autoantibody production. Some patients produce antibodies to a subset of T cells which regulates T cell but not B cell immunity. Such patients are less ill than those which produce antibodies capable of interfering with regulation of B cell function.